The class of instruments used for measurement and recording of data expressed as voltage, current, etc. and including such devices as chart recorders, digital voltmeters and the like, usually have input means for selecting one of a plurality of input ranges. The most common means is a manually switched series of voltage dividers which may be in combination with an input amplifier the gain of which is set by said switching. Switching may also be effected automatically by auto-ranging circuits which select a gain which keeps the displayed readout value within display limits, or in the case of a digital display, shift the decimal point appropriately. To maintain accuracy from range to range, each range requires a divider branch to establish a voltage or gain ratio appropriate to the particular range. Where a number of ranges are provided in a multirange instrument the values of the ratios involved may be difficult to provide with low cost resistors of standard commercial values and the number of special valued, high cost resistors may be undersireably large.
The number of special resistors required may be reduced by devising economical switched resistor networks wherein commercial valued resistors may be used, each resistor being in circuit for more than one selected range. However, the approximate ratios so obtained for the several switching modes are unlikely to be exactly the desired ratio values, but deviate therefrom by a significant amount. This situation is aggravated by the wide tolerances of low cost resistors.
Therefore it is an objective of the invention to provide the improvement of a microprocessor controlled correction to each of said approximate ratios of such an economical switched circuit to obtain the desired exact ratio.
It is a further objective of the invention to have the same microprocessor direct the requisite switching for each range entirely through the use of solid state components so as to avoid the problems associated with moving contacts, relays and the like.
Another objective of the invention is to use the same microprocessor for correction and switching that is responsible for other measurement, computation and control functions of the instrument utilizing my invention.
Yet another objective is to provide internal means whereby the necessary factor for use in the routine for ratio correction may be made available to the microprocessor.